US 7532693 B1 Abstract A method for estimating symbol timing of a guard interval in an orthogonal frequency division multiplexing receiver of a wireless local area network comprises receiving a preamble including a plurality of short training symbols; sampling said short training symbols of said preamble at a first rate; correlating a first short training symbol with a second short training symbol that is adjacent to said first short training symbol and generating a correlation signal; normalizing said correlation signal to generate a normalized correlation signal; and calculating a mean absolute difference of said normalized correlation signal.
Claims(25) 1. A method for estimating symbol timing of a guard interval in an orthogonal frequency division multiplexing receiver of a wireless local area network, comprising:
receiving a preamble including a plurality of short training symbols;
sampling said short training symbols of said preamble at a first rate;
correlating a first short training symbol with a second short training symbol that is adjacent to said first short training symbol and generating a correlation signal;
normalizing said correlation signal to generate a normalized correlation signal; and
calculating a mean absolute difference of said normalized correlation signal.
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10. A symbol timing estimator of a guard interval for an orthogonal frequency division multiplexing receiver of a wireless local area network, comprising:
receiving means for receiving a preamble including a plurality of short training symbols;
sampling means for sampling said short training symbols of said preamble at a first rate;
correlating means for correlating a first short training symbol with a second short training symbol that is adjacent to said first short training symbol and generating a correlation signal;
normalizing means for normalizing said correlation signal to generate a normalized correlation signal; and
first calculating means for calculating a mean absolute difference of said normalized correlation signal.
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18. A symbol timing estimator of a guard interval for an orthogonal frequency division multiplexing receiver of a wireless local area network, comprising:
a receiver that receives a preamble including a plurality of short training symbols and samples said short training symbols of said preamble at a first rate;
a correlator that correlates a first short training symbol with a second short training symbol that is adjacent to said first short training symbol and generating a correlation signal;
a normalizer that normalizes said correlation signal to generate a normalized correlation signal; and
a first calculator that calculates a mean absolute difference of said normalized correlation signal.
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Description This application is a divisional of U.S. Ser. No. 10/067,556, filed Feb. 4, 2002, which application claims the benefit of U.S. Provisional Application No. 60/273,487, filed Mar. 5, 2001, the entire contents of which are hereby incorporated by reference. The present invention relates to receivers, and more particularly to receivers that measure carrier frequency offset, symbol timing and/or phase noise of an orthogonal frequency division multiplexing signal. A wireless local area network (WLAN) uses radio frequency (RF) signals to transmit and receive data between electronic devices. WLANs provide all of the features and benefits of traditional hard-wired LANs without requiring cable connections between the devices. In WLANs, transmitters and receivers (often implemented as wireless network interface cards) provide a wireless interface between a client and a wireless access point to create a transparent connection between the client and the network. Alternately, the WLAN provides a wireless interface directly between two devices. The access point is the wireless equivalent of a hub. The access point is typically connected to the WLAN backbone through a standard Ethernet cable and communicates with the wireless devices using an antenna. The wireless access point maintains the connections to clients that are located in a coverage area of the access point. The wireless access point also typically handles security by granting or denying access. IEEE section 802.11(a), which is hereby incorporated by reference, standardized WLANs that operate at approximately 5 GHz with data speeds up to 54 Mbps. A low band operates at frequencies from 5.15 to 5.25 GHz with a maximum power output of 50 mW. A middle band operates at frequencies from 5.25 to 5.35 GHz with a maximum power output of 250 mW. A high band operates at frequencies from 5.75 to 5.85 GHz with a maximum power output of 1000 mW. Because of the high power output, wireless devices operating in the high band will tend to include building-to-building and outdoor applications. The low and middle bands are more suitable for in-building applications. IEEE section 802.11(a) employs orthogonal frequency division multiplexing (OFDM) instead of direct sequence spread spectrum (DSSS) that is employed by IEEE section 802.11(b). OFDM provides higher data rates and reduces transmission echo and distortion that are caused by multipath propagation and radio frequency interference (RFI). Referring now to One important task of the OFDM receiver is the estimation of symbol timing and carrier frequency offset. Symbol timing is needed to determine the samples of each OFDM symbol that correspond to the guard interval and the samples that are used for fast Fourier transform (FFT) processing. Compensation of the carrier frequency offset is also needed to maximize signal amplitude and minimize inter-carrier interference (ICI). Conventional symbol timing circuits correlate two halves of a single OFDM training symbol whose duration is equal to the duration of the data symbols. For example, see the symbol timing circuit disclosed in T. Schmidl and D. C. Cox, “Robust Frequency and Timing Synchronization for OFDM”, IEEE Trans. Commun., vol. 45, no. 12, (December 1999), pp. 1613-1621, which is hereby incorporated by reference. The conventional symbol timing circuit exhibits a plateau when there is no intersymbol interference (ISI). The duration of the plateau is the duration of the guard interval that is not affected by ISI. The plateau in the conventional symbol timing circuit corresponds to the range of acceptable times for the start of the frame. For example, the center of the plateau is a desirable estimate of the symbol timing. Since only one training symbol is employed, the conventional symbol timing circuit does not allow time for possible switching of antennas and corresponding AGC settling during packet detection. A system and method according to the invention estimates carrier frequency offset in an orthogonal frequency division multiplexing receiver of a wireless local area network. Short training symbols of a preamble of a data packet are sampled to generate a received signal. Sign bits of real and imaginary components of the received signal are quantized. In other features, the sign bits of at least two adjacent short training symbols are used to generate a correlation signal. A filtered sum of an absolute value of a real component of the correlation signal and an absolute value of an imaginary component of the correlation signal are generated. In still other features, a local maximum value of the filtered sum is identified during the short training symbols. The local maximum value is identified by updating and storing the filtered sums and by comparing at least one filtered sum to a prior filtered sum and to a subsequent filtered sum. In still other features, the local maximum value of the filtered sum is multiplied by a threshold value to identify a right edge of a plateau. A right time index value corresponding to the right edge is identified. Symbol timing of long training symbols is calculated from the right time index value. In still other features, a maximum value of the filtered sum is identified during the short training symbols. The maximum value is identified by updating and storing the filtered sums and by comparing at least one filtered sum to a prior filtered sum and to a subsequent filtered sum. A time index value corresponding to the maximum value is identified. A correlation signal value corresponding to the time index value is identified. An imaginary component of the correlation signal value corresponding to the time index value is calculated. A real component of the correlation signal value corresponding to the time index value is calculated. The imaginary component is divided by the real component to generate a quotient. An arctangent of the quotient is calculated to generate a coarse carrier frequency offset estimate. In other features of the invention, a system and method estimates fine carrier frequency offset in an orthogonal frequency division multiplexing receiver of a wireless local area network. A symbol timing estimate is generated that identifies a start time of first and second long training symbols of a preamble of a data packet. The first and second long training symbols of the preamble are used to generate a received signal. The first and second long training symbols are correlated to generate a correlation signal. A fine carrier frequency offset is calculated from the correlation signal. In yet other features, the step of calculating includes calculating imaginary and real components of the correlation signal. The imaginary component is divided by the real component to generate a quotient. An arctangent of the quotient is calculated to generate the fine carrier frequency offset estimate. In other features of the invention, a system and method updates channel estimates in an orthogonal frequency division multiplexing receiver of a wireless local area network. The channel estimates are generated for an orthogonal frequency division multiplexing subcarrier as a function of subcarrier index values. A complex number is generated by summing a product of frequency domain signals and the channel estimates for each of the subcarrier index values and dividing the sum by a sum of a squared absolute value of the channel estimate for each of the subcarrier index values. The complex number is multiplied by the channel estimates to generate said updated channel estimates. In still other features of the invention, a system and method adapt a carrier frequency offset estimate in an orthogonal frequency division multiplexing receiver of a wireless local area network. Channel estimates are generated for an orthogonal frequency division multiplexing subcarrier as a function of subcarrier index values. A complex number is generated by summing a product of frequency domain signals and the channel estimates for each of the subcarrier index values. The sum is divided by a sum of a squared absolute value of the channel estimate for each of the subcarrier index values. An imaginary component of the complex number is calculated. In yet other features, the imaginary component is multiplied by an adaptation parameter to generate a product. The product is added to a carrier frequency offset estimate to produce an adapted carrier frequency offset estimate. Further areas of applicability of the present invention will become apparent from the detailed description provided hereinafter. It should be understood that the detailed description and specific examples, while indicating the preferred embodiment of the invention, are intended for purposes of illustration only and are not intended to limit the scope of the invention. The present invention will become more fully understood from the detailed description and the accompanying drawings, wherein: The following description of the preferred embodiment(s) is merely exemplary in nature and is in no way intended to limit the invention, its application, or uses. Referring now to An output of the interleaver and symbol mapper Referring now to A cyclic prefix remover A channel estimator Referring now to After packet detection and AGC settling, the following quantities are computed for estimation of OFDM symbol timing:
r _{n})]+jsgn[ℑ(r _{n})] P _{n})|+|ℑ(P _{n})|)Where L=T _{n }contains sign bits of real and imaginary components of the received signal r_{n}. Quantization simplifies the hardware processing for symbol timing acquisition. P_{n }represents a correlation between two adjacent short training symbols of q_{n}. M_{n }represents a filtered version of |(P_{n})|+|ℑ(P_{n})|. The filter is preferably a single pole filter with a pole α_{s}. A typical value of α_{s }is α_{s}=1−3/32, although other values may be used.
Referring now to After AGC settling, P A time index n P _{n,max})]/(L)A coarse frequency correction e ^{−jω}Δ^{n }is applied to the received signal. The symbol timing is then estimated by n_{g}′=n_{g}−n_{Δ}. A typical value for n_{Δ} is n_{Δ}=32.
Referring now to An output of the metric calculator Referring now to If true, control sets M IEEE section 802.11(a) specifies that the transmit carrier frequency and sampling clock frequency are derived from the same reference oscillator. The normalized carrier frequency offset and the sampling frequency offset are approximately equal. Since carrier frequency acquisition is usually easier than sampling period acquisition, sampling clock recovery is achieved using the estimate of the carrier frequency offset ω The coarse frequency estimate ω C _{L})]where The residual frequency offset and phase noise are tracked during the data portion of the OFDM packet. Ĥ The carrier frequency estimate ω where β is an adaptation parameter and the subscript l represents values during the l-th OPDM data symbol. A typical value of β is β=1/1024. The sampling clock frequency is also adapted accordingly. Since the guard interval Referring now to An output of the divider Referring now to Referring now to In an alternate method for calculating coarse frequency according to the present invention, after packet detection and AGC settling, the following quantities are computed for estimation of OFDM symbol timing: M The duration of the plateau After packet detection and AGC settling, M The carrier frequency offset Δf is estimated by:
P _{nc})]-
- Δf=α/(2πT
_{short}) which is valid if |Δf|<1/(2T_{short}). For example, |Δf|<1/(2T_{short})=625 kHz for T_{short}=0.8 μs. The estimate of the carrier frequency offset Δf may be refined using a correlation of the two long training symbols after the sample timing is determined as will be described below.
- Δf=α/(2πT
In order to detect the falling edge of the plateau of M Since the guard interval The identification of the precise time that M IEEE section 802.11(a) specifies that the transmit frequency and sample clock frequency are derived from the same reference oscillator. Therefore, the normalized carrier frequency offset and sampling period offset are approximately equal. Since carrier frequency acquisition is more simple than sampling period acquisition, sampling clock recovery is achieved using the estimate of the carrier frequency offset. The initial carrier frequency offset estimate Δf During the OFDM data symbols that occur after the long training symbols Where β is a loop parameter. This method is currently being used with a zero order hold after IFFT in the transmitter Those skilled in the art can now appreciate from the foregoing description that the broad teachings of the present invention can be implemented in a variety of forms. Therefore, while this invention has been described in connection with particular examples thereof, the true scope of the invention should not be so limited since other modifications will become apparent to the skilled practitioner upon a study of the drawings, the specification and the following claims. Patent Citations
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